Using Naturally Occurring Tracers to Quantify Components of Urban and Agricultural Streamflow

Warming temperatures, increasing evapotranspiration, and a transition to a lower snow-to-rain ratio due to climate change is anticipated to reduce baseflow in surface waters draining watersheds in the Intermountain West; impacting both stream flow and water quality in rivers and streams. These impacts are of particular concern in urban and agricultural areas that are subject to frequent diversion as well as a wide variety of point and non-point source inputs that alter surface and subsurface exchanges along river corridors. These water sources may become the predominant component of streamflow under reduced baseflow conditions, but may be lost due to changes in urban or agricultural management practices, further impacting surface water quantity and quality. Therefore, there is a clear need to identify and quantify sources and extractions and to estimate surface and subsurface exchanges in rivers impacted by agricultural and urban areas. To identify and quantify these different sources, principal component analysis and hierarchical clustering analysis of major solutes were used to identify constituent concentration ranges representative of unique water sources. A combined flow and mass balance approach was then used to quantify the portion of streamflow attributable to each source. Within an 8 km urban reach and a subsequent 5 km agricultural reach of the Logan River in Northern Utah, we found that urban and agricultural inflows replaced snowmelt-sourced baseflow as the primary source of instream flow during low flow conditions when upstream diversions significantly depleted streamflow. These findings suggest reduction in baseflow due to climate change and changes in management practices will likely further impact instream flows increasingly sustained by urban and agricultural surface and subsurface return flows. It also emphasizes the hydrologic connectivity of urban and agricultural areas to nearby rivers and the associated challenges for water managers in maintaining flow, water quality, and aquatic habitat.